Predictors of outcome in cryptoglandular anal fistula according to magnetic resonance imaging: A systematic review

Abstract Background and Aims Anal fistula (AF) with cryptoglandular origin tends to recur, and multiple risk factors are implicated. Recently, some magnetic resonance imaging (MRI) findings with predictive value for disease outcomes have been proposed. These intrinsic anatomic features include those of the AF and its surrounding structures. This study aims to clarify the prognostic role of MRI in AF. Methods We performed a systematic search of PubMed, Embase, and EBSCO databases. Two independent reviewers conducted the search and screened the articles. We selected studies that used MRI to assess AF and reported its relationship to disease outcome. We extracted data regarding the study design, type of intervention, outcome, MRI‐measured items, and their significance. Results Out of 1230 retrieved articles, 18 were eligible for final inclusion, and a total of 4026 patients were enrolled in the selected studies. For preoperative MRI, the significant items affecting the outcome were the length of the fistula, horseshoe type, presence of multiple tracts, supralevator extension, and apparent diffusion coefficient (ADC) value. Other studies investigated the healing process using postoperative MRI. Conclusion This review found that MRI can be useful in the management of AF, both preoperatively and postoperatively. Factors, such as fistula length, horseshoe type, presence of multiple tracts, supralevator extension, and ADC value were found to be significantly associated with treatment outcomes. The presence of the fistula tract and the development of new abscesses on postoperative MRI was found to hinder the healing process. Further studies are needed to confirm these findings.


| INTRODUCTION
Fistulas are abnormal tracts or cavities that connect two epithelialized surfaces, usually between the mucosal surfaces and the skin. Anorectal or anal fistula (AF) consists of the tract(s), and internal and external openings and affect the anal canal and the perineum. 1,2 The success rate for different surgical and nonsurgical options for AF treatment varies, and failure is common. Thus, patients and treating clinicians may struggle with recurrent fistula.
Recurrence happens when the fistula reappears within 1 year after the surgical intervention. 3,4 Across the literature, the reported recurrence rates range between 5% and 69% with cutting seton fistulotomy and fibrin sealants injections, respectively. 5,6 Several risk factors contribute to the recurrence of fistula. Examples of these risk factors include high daily salt intake, metabolic syndrome, prior surgery, and a sedentary lifestyle. 7 Diagnosis is based on history and physical examination. The external opening and drainage might be visible in physical examination. 8 The anatomic relation of the fistula to the surrounding structures is confirmed with preoperative imaging techniques, such as endoanal ultrasound, computerized tomography scan, and especially magnetic resonance imaging or MRI. 9,10 Several treatment options with varying success rates are available for AF. The main treatment options are as follows: fistulotomy (the mainstay of therapy for simple fistulas), fistulectomy, fibrin glue injection, ligation of the intersphincteric fistula tract (LIFT procedure), and the advancement flap. 11,12 Treatment goals are mainly directed at wound healing, prevention of recurrence, and incontinence. 13 Regarding the prognosis, MRI is useful in two ways: First, it most accurately demonstrates the anatomic location of fistula components and determines the accompanying structures, such as abscesses. 14,15 Second, MRI is more feasible to assess the fistula's inflammatory activity which is probably associated with postoperative recurrence. 16 As stated before, preoperative MRI is done in quite a few patients with AF to aid in diagnosis and treatment. Few studies have directly investigated its role in predicting the long-term outcome of treatment using both anatomic items and fistula activity. [16][17][18] Furthermore, less information is available regarding the predictive value of postoperative MRI. 19 Hitherto, to our knowledge, there is no existing systematic review to assess and compare the findings of these studies altogether. Thus, this study is designed to clarify MRI's prognostic role in AF.
F I G U R E 1 Preferred Reporting Items for Systematic Reviews and Meta-Analyses flow diagram of the study selection process.

| Study design
We conducted the present systematic review according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guideline. The review protocol is registered in PROSPERO and is available online (CRD42022385181). A literature search was performed in PubMed and Embase, and EBSCO databases with the search terms "rectal fistula" in combination with "treatment outcome" in combination with "Magnetic resonance imaging" from inception to February 2023. We used Medical Subject Headings terms in PubMed, Emtree terms in Embase, and other keywords to find the relevant papers as outlined in Appendix. The search results were not restricted by date but were limited to the English language. We also did a complementary search in Google Scholar and hand searched the references of each selected study to avoid missing potentially relevant articles for this review.

| Eligibility criteria and quality assessment
Prospective or retrospective studies that reported the association between MRI-measured items and treatment outcome/failure in cryptoglandular AF were selected for final evaluation.
The studies that were primarily concerned with fistulizing Crohn's disease, various malignancies, animal models, use of ultrasound in the AF, and rectovaginal fistula were discarded.
Also, abstracts, reviews, ongoing studies, commentaries, letters, and editorials were excluded. We used the Newcastle-Ottawa quality assessment form for cohort studies to evaluate the selected papers. Accordingly, qualified articles had to have fair and good quality. 20

| Screening of the studies
The results of our preliminary search were assessed independently by two evaluators. First, we did a search for the duplicates and evaluated the studies by title according to inclusion and exclusion criteria. Afterward, the selected articles were assessed based on the abstract, and in case of disagreement, full-text articles were retrieved and examined. We contacted the corresponding author for a copy if the full text was unavailable. If needed, a decision was made by consensus or by a third reviewer. We extracted the study design, study set, total number of patients, type of intervention, and followup period in each study. Also, the studied outcome(s), MRI-measured T A B L E 1 Study design and characteristics including type of intervention, total number of patients, and follow-up period.

ID Study
Country Study design It is important to note that the inflammatory response following T A B L E 2 Summary of the studied items by MRI in selected studies.

Feature Details
Type of fistula T A B L E 3 Treatment outcomes in the included studies, and significant MRI-measured items.